Method for fabricating optical devices by assembling multiple wafers containing planar optical waveguides

ABSTRACT

A method for fabricating optical devices comprises the steps of preparing a first substrate wafer with at least one buried optical waveguide on an approximately flat planar surface of the substrate and a second substrate wafer with at least a second buried optical waveguide. The waveguides so formed may be straight or be curved along the surface of the wafer or curved by burying the waveguide at varying depth along its length. The second wafer is turned (flipped) and bonded to the first wafer in such a manner that the waveguides, for example, may form an optical coupler or may crossover one another and be in proximate relationship along a region of each. As a result, three dimensional optical devices are formed avoiding conventional techniques of layering on a single substrate wafer. Optical crossover angles may be reduced, for example, to thirty degrees from ninety degrees saving substrate real estate. Recessed areas may be provided in one or the other substrate surface reducing crosstalk in a completed three dimensional crossover device. Three dimensional optical couplers may comprise waveguides of identical or dissimilar characteristics. Moreover, three dimensional optical switches may be formed using the proposed flip and bond assembly process.

[0001] This application is a National Filing pursuant to 35 U.S.C. 371based upon International Application No. PCT/US01/27393, filed Sep. 4,2001, which claims priority to U.S. Provisional Application No.60/230,205, filed Sep. 5, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] The present invention relates to the field of fabricating opticaldevices and, more particularly, to a method for fabricating opticaldevices, including, for example, couplers and crossovers, byindividually assembling multiple wafers, each containing at least oneplanar optical waveguide (POW), and bonding the wafers together to formsaid optical devices and the optical devices fabricated by such a methodof fabricating.

[0004] 2. Description of the Related Arts

[0005] The defining element of planar optical waveguide (POW) technologyis the optical waveguide itself, a region of relatively high refractiveindex which is typically much longer than it is wide or deep, formed,for example, by diffusion on a substrate wafer of lower refractive indexmaterial. Light is bound to the high-index guide, eliminating thenatural diffraction spreading of a free light beam and allowing thelight to be turned through curving paths.

[0006] Typically, the waveguides are fabricated near the (approximatelyplanar) surface of the substrate wafer, like the interconnections in anelectronic integrated circuit.

[0007] Current planar waveguide technology has been focused on a singlelayer of waveguides, which, for example, severely limits crossing ofwaveguides. A few research results have explored three dimensionalcouplers in semiconductor materials, but that work requires multipleepitaxial growth processes of layers on a single wafer, an expensiveprocess which has not been extended to glass waveguides. Presently, asmany as seven layers may be grown on a substrate with differentproperties, but with each additional layer, complications in tolerancesresult. Recently, Raburn et al. in their paper Double-Bonded InPlInGaAsPVertical Coupler 1:8 Beam Splitter, published in “IEEE PhotonicsTechnology Letters”, Vol. 12, December, 2000, disclose a three layerdouble bonding process where the three layers are grown using vapordeposition. A first wafer with etched waveguides is bonded to a blanksecond wafer. The second wafer is then etched and a third wafer bonded.A waveguide of one layer may be at a different height than anotherwaveguide of another layer.

[0008] Direct wafer bonding processes are known, for example, from Zhuet al., Wafer Bonding Technology and its Application in OptoelectronicDevices and Materials, published in “IEEE Journal of Selected Topics inQuantum Electronics,” vol. 3, pp. 927-936, 1997 and from Black et al.,Wafer Fusion: Materials Issues and Device Results, published in IEEEJournal of Selected Topics in Quantum Electronics,” vol. 3, pp. 943-951,1997.

[0009] Consequently, there remains a need in the art for improvedmethods of fabricating optical devices and, in particular, couplers andcross-overs having different properties than conventional opticaldevices.

SUMMARY OF THE INVENTION

[0010] We propose to individually assemble and bond two planar waveguidesubstrate wafers together according to the desired optical device. Thesurfaces of the individual substrate wafers are placed and then bondedface-to-face according to known bonding techniques to fabricate thedesired optical device so that the waveguides are in appropriate,proximate relationship to one another depending on the device to befabricated. Where waveguides cross each other at an angle forming acrossover, an interconnection crossover between waveguides results, withmuch better isolation and lower crosstalk than the crossthroughsachieved in conventional single-wafer technology. Also, it will be shownthat wafer real estate may be preserved in crossovers as the crossoverangle may be substantially reduced from, for example, ninety degrees toa substantially lesser angle such as thirty degrees.

[0011] Where waveguides are at least in part constructed with parallelsegments on the substrate wafers, for example, forming a coupler,overlap of optical modes creates a three-dimensional (3D) opticalcoupler linking the upper waveguide of one substrate wafer and a lowerwaveguide of a second substrate wafer. (By parallel segments is intendedthe construction of waveguide segments in a line whereby the two linesegments are in the same plane and the waveguide segments are opticallycoupled.) By appropriate design of the waveguides and one or moreinteraction regions, the energy transfer may be complete or partial,wavelength-selective or broadband, polarization-selective orpolarization-independent. The proposed method of fabrication mayfabricate couplers between dissimilar waveguides, such as a pump couplerwhich directly connects an undoped waveguide carrying pump light to anErbium (Er)-doped waveguide which amplifies signal light.

[0012] The invention, for example, will enhance the optical transportand routing systems at the heart of a telecommunications network to bemade smaller, cheaper, more reliable and more capable. In particular,Er-doped waveguide amplifiers (EDWAs) fabricated according to theproposed method may replace bulkier Er-doped fiber amplifiers intransport systems in which three dimensional couplers fabricatedaccording to the present invention may be used to improve the gain,power efficiency, and wavelength flatness of existing EDWAs. ImprovedEDWAs should also play an important role inside optical switches androuters, to overcome the loss of passive components. Three dimensionaloptical couplers may also serve as channel selection and noise reductionfilters inside highly integrated DWDM systems. Also, the high-isolationcrossovers enabled by the proposed wafer bonding process may enablesophisticated routing photonic integrated circuits (PICs), eliminatingmuch of the fiber congestion and complexity associated with backplanesand front panels of current DWDM equipment. The present invention willallow PICs to be simultaneously more compact and more capable. Also, thephotolithography needed for fabrication of 3D couplers by wafer assemblyis less demanding than that needed for single layer couplers. Finally,the creation of couplers between dissimilar waveguides, difficult orimpossible with single-layer techniques, is practical with waferassembly. Glass or silica waveguides, which are preferred for EDWAs andmost passive devices, are easily accommodated by the present technique.

[0013] Planar optical waveguide (POW) technologies and fabrication inaccordance with the proposed process enable a broad range of opticaldevices by routing light along controlled paths near the surface of asubstrate wafer. In some cases, the routing is simple, such as aFabry-Perot laser, which guides light back and forth through anamplifying region until feedback causes a controlled oscillation. Inothers, the routing may be quite complex, involving switching and/ormultipath interferometry. Although similar functions can often beachieved with fiber- or free-beam technologies, planar waveguideprovides the most compact implementation of complex devices, with thegreatest mechanical stability.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1(a) shows a conventional diffusion method of fabricating aplanar optical waveguide on a substrate wafer, depth of the waveguide 15being determined by diffusion time, temperature, electric field densityand other factors; FIG. 1(b) shows the result of burying a waveguidesegment more deeply in the center of a substrate wafer than the pointsof entry at either end of a waveguide.

[0015]FIG. 2 show three dimensional wafer bonding assembly according tothe present invention, for example, for fabricating a waveguidecrossover wherein FIG. 2(a) shows the separate preparation of two waferseach having a waveguide diffused thereon, and a direction of flipping tofabricate a three dimensional crossover shown in isometric view whileFIG. 2(b) shows the additional step of etching recessed areas in thevicinity of each crossing waveguide to improve isolation and crosstalkperformance.

[0016]FIG. 3 shows a conventional process for fabricating a simpletwo-mode optical coupler utilizing single-layer planar optical waveguidefabrication wherein FIG. 3(a) provides an isometric view and FIG. 3(b) atop view.

[0017]FIG. 4 shows a three dimensional wafer bonding assembly accordingto the present invention, for example, for fabricating a threedimensional optical coupler wherein FIG. 4(a) shows, for example, thediffusion of identical waveguides on identical substrates and a flippingof one on to the other, forming the isometric view (FIG. 4(b)) and topview (FIG. 4(c)) of a three dimensional coupler fabricated according tothe principles of the present invention.

[0018]FIG. 5 shows the fabrication of an optically triggered waveguideswitch in accordance with the present invention.

[0019]FIG. 6 shows construction of a typical crossover angle ofapproximately ninety degrees in FIG. 6(a) and how a substantially lesserangle, for example, a thirty degree crossover angle may be achieved andwafer real estate saved in FIG. 6(b) utilizing a variant of the methodof fabricating an optical crossover of FIG. 2; FIG. 6(c) is analternative embodiment in cross-sectional view of a device fabricatedaccording to the present invention in which waveguides are implantedhaving varying depth in the planar substrate as taught in FIG. 1(b),thus forming a crossover of waveguide segments by varying the depth oftheir fabrication. In the design of FIG. 6(c), a substantially reducedcrossover angle may also be achieved.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0020]FIG. 1(a) shows a conventional diffusion method of fabricating aplanar optical waveguide 15 on a substrate wafer 10, the depth of thewaveguide 15 being determined by diffusion time, temperature, electricfield density as well as other known factors. A defining element ofplanar optical waveguide (POW) technology is the optical waveguideitself, a region 15 of relatively high refractive index which istypically much longer than it is wide or deep, surrounded by wafersubstrate material of lower refractive index. Light is bound to thehigh-index guide 15, eliminating the natural diffraction spreading of afree light beam and allowing the light to be turned through curvingpaths as necessary. Typically, the waveguides are fabricated near the(approximately flat and planar) surface 12 of a substrate wafer 10, likethe interconnections in an electronic integrated circuit. (However, perFIG. 1(b) and FIG. 6(c), some embodiments may involve burying thewavguides more deeply, for example, at the center of the substrate waferthan at the points of entry to intentionally build a crossover or otheroptical device.) Waveguides 15 may be fabricated, for example, bydeposition and etching, ion implantation, diffusion techniques, or otherknown techniques, and the materials forming the wafer substrate 10 maybe semiconductors, crystalline dielectrics, glasses, or polymers, amongother known materials. Although many implementations are possible, onepreferred embodiment may comprise diffused waveguides in glass, aneconomical technology with some specific advantages discussed furtherbelow.

[0021] Following from left to right in FIG. 1(a), formation of diffusedwaveguides 15 begins with preparation of a substrate wafer 10 withcontrolled properties at surface 12. Patterned areas (typically straightor curved stripes) of a diffusion source material 13, which may bedopant, are deposited onto the approximately flat, planar surface 12.Then the composite structure 10 is subjected to heat and/or an electricfield in a controlled manner, allowing dopant ions to diffuse from thesource layer into the substrate 10 and producing regions of increasedrefractive index where the diffusion source overlay 13 is present. Thewaveguide depth and the lateral spreading at the pattern edges aredetermined by the appropriate choice of materials and processconditions. Electric fields may be applied during the process to obtainmore flexible control of vertical and horizontal dopant profiles. Dopantin-diffusion is a simple process, adaptable to small- or large-scaleproduction of optical waveguides 15 and has been commercially applied tolithium niobate optical modulators. Other suitable processes may beemployed to advantage depending on the desired optical device. At theright of FIG. 1(a) is shown the wafer 10 with waveguide 15 implanted butwith dopant source material still present. Finally, the dopant sourcematerial is removed and the top surface planed leaving the wafer 10complete with planar optical waveguide 15 implanted. That is, thewaveguide lies in a plane parallel to the approximately flat planarsurface of the substrate wafer. As already indicated, the finishedproduct is shown in FIG. 1(a) with a straight waveguide 15 appearing tobe at even depth. In fact, by varying the initial location of dopantsource material, degree of heat and field intensity, curvature and depthand other measurable characteristics of the formed waveguide 15 may bevaried as shown in FIG. 1(b). In this figure, the waveguide isintentionally buried more deeply in the center of the wafer withsubstrate material covering the waveguide in the center. Thus, thewaveguide may follow a, for example, vertically curved path through thesubstrate.

[0022] Referring to FIG. 2, there is shown the process of fabricating,for example, a three-dimensional optical crossover according to thepresent invention which employs the conventional processes of FIG. 1 inpreparing at least first and second wafer substrates. Moving from leftto right in FIG. 2(a), a wafer substrate 10 has diffused or otherwisedeposited therein at least one waveguide 15 formed on or near preferablyplanar surface 12. (Similar reference numerals have been used betweenFIGS. 1 and 2 to refer to similar elements since FIG. 2 relies in parton POW waveguide formation as per FIG. 1). A second substrate wafer 20may be similarly formed having one or more waveguides 25 diffused orotherwise implanted therein. The first wafer substrate 10 may be thenused as a stationary substrate for receiving a flipped wafer substrate20 as a second layer of a completed device to form a completed threedimensional optical crossover 30 (shown to the right of FIG. 2(a). In analternative embodiment and to improve crosstalk isolation and per FIG.2(b), recessed areas 18 and/or 28 may be formed in one or the otherwafer substrate 10 or 20 or both prior to flipping and bonding. If tworecessed areas 18, 28 are formed, they may preferably match afterflipping. When a recess is formed in one or the other or bothsubstrates, the recess(es) will form a void which may be filled tocontain inert gas, a vacuum or other desired medium as will be discussedfurther below. As will be further discussed in connection with FIG. 6,the crossover angle may be reduced from ninety degrees to a much smallerangle, for example, thirty degrees, more or less, using the proposedmethod of fabrication of the present invention saving wafer real estate.

[0023] A key building block of POW devices is the conventional two-modecoupler shown in FIG. 3. At the opposite edges of the substrate, wherethe two waveguides 32 and 34 are well-separated on substrate wafer 38,there is negligible overlap of the optical modes, and no energy istransferred from one waveguide to the other. In contrast, mode overlapis stronger in a central interaction region 36, leading to coupling ofthe waveguide modes and a predictable shift of light from one waveguideto the other. By appropriate design of the waveguides and theinteraction region, the energy transfer may be complete or partial,wavelength-selective or broadband, polarization-dependent or not. If theinteraction region can be controlled by a parameter such as temperature,application of light or electric field, a switch may be realized. Inmost POW devices, these optical couplers are laid out laterally, betweentwo waveguides at the same constant depth relative to the planarsurface. The fabrication process of FIG. 4, as discussed below, providesan alternative, vertical or three-dimensional (3D) coupler in which notall waveguides fall within a single plane but are closely proximate toone another and may lie in proximate parallel planes.

[0024] Vertical or 3D couplers in semiconductor materials have beenconstructed previously by fabricating two layers of waveguides on asingle substrate. Up to seven layers are known. In contrast, we proposethat two or more substrates, each prepared with a single layer ofwaveguides, be juxtaposed so that the optical modes of waveguides ondifferent wafers overlap with each other. A practical, stable means ofassuring the required intimate contact is the use of direct waferbonding as introduced in the Description of the Related Arts sectionabove or in accordance with other well known bonding techniques. Theprocess may use wafer substrates with smooth, clean surfaces that may betreated with heat and pressure to form a strong, adhesive-free bond.Wafer bonding has been demonstrated with a wide variety of matched andmismatched substrates, including commercial fabrication ofsilicon-on-insulator wafers. The proposed assembly is illustrated inFIGS. 2 (cross-over), 4 (coupler) and 5 (a waveguide switch) among otherpossible devices.

[0025] Referring to FIG. 4, there is shown a method of fabricating athree dimensional optical coupler. A first substrate wafer 200 hasformed thereon a first curved waveguide 210 along the approximatelyflat, planar surface of wafer 200. A second identical wafer 220 has asecond waveguide 230 formed therein in the same manner. Of course, theforming of identical waveguides on identical wafers has some economicadvantages over forming differently shaped waveguide(s) on the secondwafer, but such is possible within the principles of the presentinvention. As in FIG. 2, the second wafer 220 is flipped so that theformed waveguides are in close proximity after flipping and bonded tothe first wafer 200 forming a completed three dimensional coupler shownin isometric view in FIG. 4(b) and in top view in FIG. 4(c) having acentral coupling region.

[0026] The depicted, wafer-bonded 3D coupler of FIG. 4(b) or (c) canenable a wide variety of novel and capable devices. One major advantageof the process is its ability to construct couplers between radicallydifferent waveguides, rather than identical waveguides as shown. Forexample, the waveguides 210, 230 on the two substrates 200, 220 may bederived from different dopant ions or treated at different diffusiontemperature or electric field, to obtain different optical mode shapesor propagation velocities. The implanted waveguides may lie in parallelplanes in the completed coupler device or not as the designer chooses.For example, the designer may vary the depth of implantation of thewaveguides 210, 230 along their length per FIG. 1(b) or their pathsthrough the substrate, for example, one curved, one straight or varyingthe curvature of the separate waveguide paths. The substrates into whichwaveguides are diffused may also differ. An Erbium (Er) doped waveguideamplifier (EDWA) constructed from one Er-doped waveguide, for example,waveguide 210, and one passive Er-free waveguide, for example, waveguide230, is a useful instance of this idea. The active Er-doped waveguidecan be optimized to amplify the signal light, while the Er-freewaveguide is independently optimized to transport pump light with verylow loss. Unlike conventional end-pumped EDWAs, the wafer-bonded EDWA ofthe present invention can be designed with multiple pumping locationsinside the length of the active region, to assure good noise figure andefficient pump utilization at all points in the amplifier. The use of 3Dcouplers according to the present invention is also valuable indistributing pump power to arrays of waveguide amplifiers and incombining power from redundant pumps, due to the enhanced crossovercapabilities discussed below.

[0027] In addition to its use for pump light routing, the 3D coupler ofFIG. 4 can also contribute wavelength shaping filters to EDWAs. Acoupler which extracts light at the spontaneous emission peak can beused to equalize the amplifier's gain spectrum, providing any tailoredspecific shape desired, for example, the flat wavelength response neededfor wavelength-division multiplexed systems. Mid-amplifier filteringusually provides the best noise and power efficiency, and a 3D coupleraccording to FIG. 4 offers the unique opportunity of filtering in themid-stage of the amplifier without breaking the active waveguide.

[0028] There are several ways in which 3D couplers constructed accordingto the principles depicted in FIG. 4 may enhance the performance ofwide-band EDWAs, including compound amplifiers which cover both C-band(typically from 1525-1565 nm) and L-band (typically from 1565-1615 nm).First, 3D couplers may be used to inject pump light from pump lasers attwo or more wavelengths. Second, 3D couplers may be placed to injectpump light at strategic points along an active waveguide which hasvarying dopant composition along its length. Third, wavelength-selective3D couplers may be used to route C-band and L-band signals to theappropriate paths in a compound amplifier.

[0029] In alternative embodiments, one (or more than one) opticalcrossover may enable distribution of a single pump source via opticalcoupling to not only one amplifying waveguide but two or more amplifyingwaveguides. One of ordinary skill in the art may construct a compound,complex device having signal distribution among waveguides only limitedby the imagination.

[0030] Mating of active and passive waveguides can also enable anoptically triggered waveguide switch, as shown in FIG. 5. The activewaveguides B and D, 60 and 62, are doped with saturable absorber ions.These can be pumped to a higher energy state by optical injection at theresonant wavelength, which will saturate the absorption and shift therefractive index at non-absorbing wavelengths. Thus, if waveguide B 61is optically pumped at its resonant wavelength, its refractive index atlonger, transparent wavelengths will be lower than that of unpumpedwaveguide D 62. Thus, long-wavelength light from input waveguide C 52, apassive waveguide, will be preferentially coupled into waveguide D 62.Subsequent coupling to passive waveguide E 53 can be used if it isdesirable for the output waveguide to be undoped. If the optical pumpingis moved from waveguide B 61 to waveguide D 62, the signal output can beswitched to output waveguide A 51 from waveguide E 53.

[0031] Another advantage of 3D couplers is flexible and reproducibletailoring of the coupling strength between the coupled waveguides. In aconventional lateral coupler per FIG. 3 built from a single layer ofwaveguides, the coupling strength is very sensitive to any errors inwaveguide spacing. At the same time, lithography and patterning are mademore difficult by the close approach of multiple waveguides. Incontrast, the 3D coupler of FIG. 4 allows independent lithography andpatterning of the two waveguides 210, 230, while reducing sensitivity tolateral placement errors. Realizing the 3D coupler by wafer bonding ofdiffused waveguides offers an additional degree of customization: thecoupling strength can be tuned, either before or after wafer bonding, byadjusting the diffusion parameters to control waveguide depth as shownin FIG. 1(b).

[0032] Three dimensional waveguide couplers per FIG. 4 also offerpowerful capabilities for crossovers in photonic circuits. Inconventional single-layer technology, waveguides cannot physically crossover one another, they must cross through each other, introducingcrosstalk and loss in both paths. Although these impairments can be mademanageable by relying on large intersection angles, the resultingcircuits consume a lot of expensive real estate on the wafers (FIG. 6a).FIG. 6(a) shows a conventional crossover at ninety degree angles and howwafer real estate is wasted. By coupling light into a second layer forthe crossover, it becomes possible to achieve low crosstalk and losswith much smaller crossing angles, for example, on the order of thirtydegrees, yielding a more compact, cost-effective photonic circuit (FIG.6b) using far less wafer real estate.

[0033] For even more compact circuits, it may be desirable to arrangeweaker coupling for the crossover areas than for the coupler areas.There are two possibilities for achieving this area-selective couplingstrength. First, waveguides may be vertically curved by varying depth ofimplant (or curved along the horizontal planar surface of eachsubstrate); that is, the waveguides may be implanted deeper (weakercoupling) in some regions than they are in others of the substratewafer. FIG. 6(c) is an alternative embodiment in cross-sectional sideview of a device fabricated according to the present invention in whichthe waveguides are implanted in each substrate wafer at variable depth.Variable-depth waveguides may be fabricated in each substrate byapplying localized electric fields during diffusion, by applyinglocalized regions of higher temperature (such as by laser heating), orby localized ion implantation to accelerate the diffusion process. InFIG. 6(c), a crossover angle of substantially less than 90 degrees, forexample, thirty degrees can also be achieved. Alternatively, couplingmay be weakened in regions of waveguide crossover simply by creating,for example, a rectangular recessed area 18 or 28 in one or bothsubstrates before wafer bonding, as shown in FIG. 2. After assembly asshown in either FIG. 2, the crossover areas will have local voidsbetween the upper and lower waveguides. The depicted recessed areas mayhave rectangular or curved shape and, if rectangular, are of even depth,but may be of other shape and depth according to desired results.Whether filled with vacuum, air, or an inert gas, the voids formed byrecessed area(s) 18, 28 and as shown in FIG. 2 will have a refractiveindex much lower than that of the glass substrates, ensuring aneffective decoupling in the crossover areas and improved crosstalkperformance. The recessed areas may be intentionally filled with anothermaterial or composition of solids, liquids and/or gases that may have anoptical filtering or other attenuating effect at one or morewavelengths.

[0034] As a final note, it may be understood by one of ordinary skill inthe art that the wafer-bonded 3D coupler technology is not limited totwo layers of waveguides. As has been noted in other types ofwafer-bonded devices, the bond is very strong, and it is quite practicalto polish away the body of the upper substrate, and then to bond on athird wafer, (which may be flipped or not flipped before bonding) toobtain more complex composite structures. Moreover, other embodimentsand other optical devices than those described above may be formed usingthe principles of the present invention simply described in the drawingsas a flip and bond assembly of planar optical waveguide substratewafers. All articles referenced herein should be deemed to beincorporated by reference as to any subject matter deemed essential toan understanding of the present invention and/or conventional waveguidediffusion and/or wafer bonding technology.

What we claim is:
 1. A method for fabricating an optical device, inwhich at least one wafer containing at least one buried opticalwaveguide is assembled and bonded to at least one other wafer containingat least one buried optical waveguide, the bonded wafers beingpositioned with respect to one another and forming said optical devicesuch that different regions of each waveguide are formed by the bondingto be in optical relationship to one another.
 2. The method of claim 1,in which at least two planes of waveguide are formed in each of twosubstrates parallel to one another after the wafers are bonded.
 3. Themethod of claim 1, in which at least two waveguide wafers are bonded toeach other such that substantially the extent of planar surfaces of thewafers, each having diffused thereon said at least one waveguide, are inintimate contact.
 4. The method of claim 1, in which said at least twowafers are composed of different materials.
 5. The method of claim 4, inwhich at least one of the at least two wafers is crystalline and anotherone of the at least two wafers is non-crystalline.
 6. The method ofclaim 1, in which at least two of the wafers contain waveguides withdifferent composition from each other.
 7. The method of claim 1, inwhich at least two of the wafers contain waveguides with differentrefractive index profile from each other.
 8. The method of claim 1, inwhich at least one of the wafers contains at least one waveguide capableof providing optical gain.
 9. The method of claim 8, in which an opticalgain medium is a semiconductor.
 10. The method of claim 8, in which anoptical gain medium is doped with rare earth ions.
 11. The method ofclaim 10, in which said optical gain medium is doped with erbium. 12.The method of claim 1, in which at least one of the wafers contains atleast one waveguide capable of providing saturable optical absorption.13. The method of claim 12, in which a saturable absorption medium is asemiconductor.
 14. The method of claim 12, in which a saturableabsorption medium is doped with rare earth ions.
 15. The method of claim14, in which the saturable absorption medium is doped with erbium. 16.The method of claim 1, in which respective segments of said at least twowaveguides of said at least two wafers are positioned so that theirguided waves interact to form an optical coupler.
 17. The method ofclaim 1, in which respective segments of said at least two waveguides ofsaid at least two wafers are positioned so that their guided waves crosseach other without strong interaction forming an optical crossover. 18.The method of claim 1, in which: at least two waveguide segments arepositioned so that their guided waves interact to form an opticalcoupler; and at least two waveguide segments are positioned so thattheir guided waves cross each other without strong interaction, so thatan optical crossover is formed; and at least one optical coupler and atleast one optical crossover are interconnected to form an opticalintegrated circuit.
 19. The method of claim 1, in which at least onewaveguide is fabricated by a process including dopant in-diffusion. 20.The method of claim 1, in which at least one waveguide is fabricated bya process including ion implantation.
 21. The method of claim 1, inwhich at least one waveguide is fabricated by a process including layerdeposition and etching.
 22. The method of claim 1, in which the depth ofburying at least one waveguide is varied along its length so that thewaveguide is curved with respect to the flat planar surface of thewafer.
 23. The method of claim 1, in which a region of low refractiveindex is provided between at least two waveguides to control the degreeof coupling between them.
 24. The method of claim 1, in which a regionof high refractive index is provided between at least two waveguides tocontrol the degree of coupling between them.
 25. The method of claim 1,in which a metallic region is provided between at least two waveguidesto control the degree of coupling between them.
 26. The method of claim22, in which the depth of at least one waveguide is varied along itslength to control the degree of coupling between waveguide segments ofat least two waveguides.
 27. An optical amplifier apparatus comprising:at least one waveguide capable of optical gain; and at least one opticalcoupler capable of introducing light into said amplifying waveguidewithout interrupting or terminating said amplifying waveguide in whichoptical pumping light is coupled into said at least one amplifyingwaveguide gradually, over an extended region of its length.
 28. Anoptical amplifier apparatus comprising: at least one waveguide capableof optical gain; and at least one optical coupler capable of introducinglight into said amplifying waveguide without interrupting or terminatingsaid amplifying waveguide in which optical pumping light is coupled intoat least one amplifying waveguide at at least three regions along itslength.
 29. An optical amplifier apparatus comprising: at least twowaveguides capable of optical gain; and at least one optical couplercapable of introducing light into one of said amplifying waveguideswithout interrupting or terminating said amplifying waveguide in whichat least one optical crossover is provided to enable distribution of asingle pump source to said at least two amplifying waveguides.
 30. Anoptical amplifier apparatus comprising: at least one waveguide capableof optical gain; and at least one optical coupler capable of introducinglight into said amplifying waveguide without interrupting or terminatingsaid amplifying waveguide in which at least one optical crossover isprovided to enable distribution of at least two pump sources to saidsingle amplifying waveguide.
 31. An optical amplifier apparatus,comprising: at least one waveguide capable of optical gain; and at leastone optical coupler capable of extracting light from said amplifyingwaveguide without interrupting or terminating said amplifying waveguide.32. The apparatus of claim 31, in which said at least one opticalcoupler extracts light at selected wavelengths to achieve an equalized,gain spectrum of predetermined shape for an optical amplifier.
 33. Theapparatus of claim 32 in which said gain spectrum of predetermined shapecomprises a wavelength flattened shape.
 34. An optical switch apparatuscontaining at least one waveguide capable of saturable absorption,coupled to at least one optical coupler fabricated according to themethod of claim
 16. 35. An optical switch apparatus containing at leastone optical coupler fabricated by the method of claim 16, in which therefractive index of at least one waveguide segment is controlled todetermine routing of at least one optical signal through the switch. 36.An optical switch apparatus as recited in claim 35 said control isprovided optically.
 37. An optical switch apparatus as recited in claim36 wherein said optical control is provided by at least one waveguide.38. An optical switch apparatus as recited in claim 35 wherein saidcontrol is provided electrically.
 39. A method of fabricating an opticaldevice comprising the steps of: forming at least one buried opticalwaveguide in a planar surface of a first wafer substrate; forming atleast one buried optical waveguide on a planar surface of a second wafersubstrate; placing the second wafer substrate on to the planar surfaceof the first wafer substrate and bonding the first and second wafersubstrates together.
 40. A method of fabricating an optical device asrecited in claim 39 further comprising the step of forming a recessedarea on the planar surface of one wafer substrate before bonding thefirst and second wafer substrates together.
 41. A method of fabricatingan optical device as recited in claim 39 wherein said at least oneplanar waveguide of one or the other wafer substrate is curved.
 42. Amethod of fabricating an optical device as recited in claim 39 whereinsaid at least one planar optical waveguide of said first wafer substratecrosses said at least one planar optical waveguide of said second wafersubstrate at an angle of less than fifty degrees after bonding.
 43. Amethod of fabricating an optical device as recited in claim 40 furthercomprising the step of filling said recessed area with a material otherthan one of a vacuum, air and an inert gas.
 44. A method of fabricatingan optical device as recited in claim 39 wherein one waveguide is buriedat varying depth along its length in relation to the planar surface ofits corresponding wafer substrate.